1. Field of the Invention
This invention relates generally to wear-resistant ceramic-metal composite structures. More particularly, this invention relates to an internal combustion engine component formed from a ceramic element mechanically retained within a metal element.
2. Description of Related Art
The harsh operating conditions encountered in an internal combustion engine, particularly the high temperatures and high pressures, cause engine components to wear rapidly. Mechanically driven actuators and actuating components are especially susceptible to wear in this environment. Consequently, the materials used for producing actuating engine components should provide good mechanical strength, thermal stability and wear resistance. Metals have typically been used to form such components. However, ceramics, such as zirconia, silicon nitride, silicon carbide and the like, have been found to exhibit excellent mechanical strength, thermal stability and wear resistance. As a result, ceramics are increasingly being used as structural materials for components of gas turbine engines and diesel engines.
Ceramics, despite their promise as wear-resistant engine components, are generally hard and brittle and lack the formability and workability of metals which are conventionally applied to low cost precision engine components. Composites formed from a ceramic element thickness in an elastomeric bonding layer to compensate for expansion differences in the metal and ceramic. The ceramic-metal composite disclosed in this patent still requires close tolerance machining of both the ceramic insert and the metal component to bond these elements securely together. Such precision machining is time-consuming and can increase significantly the cost of an internal combustion engine component that must be produced in this manner.
It is also known to secure a ceramic component to a metal component by an interference fit between the two components to form a composite structure useful in an internal combustion engine. U.S. Pat. No. 4,366,785 to Goloff et al, for example, discloses a tappet for an internal combustion engine with a ceramic wear resistant insert maintained within the annular metal rim of the main body of the tappet by an interference fit. The wear resistant insert is formed to be slightly larger in diameter than the diameter of the recess into which it is fitted. The ceramic insert is forced into the recess under sufficient pressure to press fit it in the tappet main body. The insert is not required to be sized to fit exactly within the recess in the tappet, but must be slightly larger than the recess. However, to provide a secure interference fit without damaging the metal or ceramic components, each must still be formed to close tolerances.
Additional examples of metal-ceramic composite bodies joined using interference fit methods are disclosed in U.S. Pat. Nos. 4,614,453 to Tsuno et al, 4,794,894 to Gill, assigned to Cummins Engine Company, Inc., assignee of the present invention, and 4,806,040 to Gill et al, also assigned to Cummins Engine Company, Inc.
U.S. Pat. Nos. 4,667,627 and 4,709,621 to Matsui et al disclose engine pares which include ceramic elements or inserts which are attached to metallic elements. Specifically, the ceramic inserts may be attached by shrink-fitting or press fitting. The ceramic may also be joined to the metal with a metallized layer of metal paste formed from a metal powder selected according to the composition of the metal used for the metal component part. Each of these methods still requires that the metal component and the ceramic insert be machined to specific tolerances, however.
Another method of securing a ceramic wear resistant element to a metal element utilizes a separate connecting element or retaining element. U.S. Pat. No. 4,325,647 to Maier et al discloses a connecting element for ceramic and metallic parts formed from an insulating resilient body of a ceramic material. The thermally or mechanically induced differences between the ceramic and metal structures are equalized, and contact stress in the operating state is limited. The insulating body positively connects the ceramic and metallic elements and operates effectively to secure these elements when it has specific physical characteristics, for example, a thermal conductivity of 0.02 to 0.25 W/cmK at a temperature difference between the ceramic and the metallic structural elements of about 100.degree. to 1500.degree. C. and an elastic modulus of about 5000 and 150,000 N/mm.sup.2. This composite, however, is not intended to be used in the interface between a mechanically driven actuator or actuating component, but in connection with a piston in the engine cylinder.
External connectors have been proposed for joining a ceramic element to a metal element. For example, U.S. Pat. No. 4,883,911 to Haahtela discloses a ceramic piston ring carrier held in place on a metal piston by casting in or with a locking ring to improve force transmission and frictional conditions between the piston and the cylinder. U.S. Pat. No. 4,848,286 to Bentz, assigned to Cummins Engine Co., also discloses the use of an external metal connector for joining ceramic and metal components of a pivot rod. Neither of these patents, however, suggests that the arrangement described therein could be used to secure a ceramic element to a metal element to form a wear-resistant interface between engine actuator and actuating components.
Metal and ceramic elements may be connected by integrally shaping each element to produce a secure bond between the ceramic and metal elements. For example, U.S. Pat. No. 4,404,935 to Kraft discloses a ceramic capped piston wherein the ceramic piston cap is fitted into a recess and joined to a metal piston by intermeshing radial flanges biased into engagement with the piston by a spring. Both the ceramic cap, which is intended to provide heat insulation rather than wear resistance, and the metal piston require special machining or casting to provide the necessary intermeshing radial flanges.
The prior art, therefore, has failed to provide a ceramic-metal composite structure in which a ceramic element is reliably and durably retained by a metal element so that the composite can be used to form a wear-resistant interface between actuating and actuator components in a internal combustion engine. The prior art has further failed to provide a ceramic-metal composite that is sufficiently reliable to be useful as a driven actuator or actuating component in an internal combustion engine that can be produced inexpensively in high volume on a large scale so that the production of such components is commercially feasible.